The use of piezoelectric material for generating electric current flow is well known in the art. Piezoelectric crystals, when compressed, generate an electrostatic output. Conversely, application of electric current across a piezoelectric crystal causes contraction of that crystal. Therefore, theoretically, an electric power supply using piezoelectric material should exhibit an enhanced simplicity of construction, requiring only a limited number of nonrotating, moving parts.
A possible use for such a piezoelectric power supply is in aircraft flight control systems. A new generation of aircraft presently under consideration will employ inherently unstable control surfaces and therefore will require a large number of sensors and control surface actuators that must be constantly monitored and controlled by a central computer to maintain stable flight. These sensors and actuators will most likely communicate with the computer through optical fibers to reduce the requirement for electrical wiring. Since optical fibers cannot provide motive electric power to these devices, there is a need for localized power sources. Ideally, such power sources should utilize available high pressure hydraulic actuator fluid, without requiring large fluid flows.
Piezoelectric material is used in prior art power supplies which require either hydraulic fluid valving or internal combustion explosions to compress the crystals. Such devices cannot be used to generate continuous power without some external means of controlling the hydraulic or combustive power oscillations. Phase generators, frequency controllers and other types of external feedback control means make these power supplies complex and unwieldy.
Other prior art piezoelectric power supplies may use fluidic oscillators that cause vibration in a mechanical reed or diaphragm with a piezoelectric element attached thereto. In these devices, the oscillations are caused by acoustic feedback within the fluidic circuit itself, such feedback being generally effected by turbulent fluid flow through restrictions or against sharp objects. The piezoelectric element acts as a passive transducer, converting these vibrations into electrical energy. These power supplies require a high fluid flow velocity to maintain usable power output levels and therefore, are unsuitable for applications where high velocity flow is unavailable or undesirable. Furthermore, such fluidic oscillator power supplies may also suffer from an impedance mismatch between the motive fluid and the piezoelectric element. Acoustic vibrations set up within the fluidic oscillator may not occur at the same frequency as the natural oscillatory frequency of the piezoelectric element. Such a mismatch in oscillatory frequency leads to inefficient conversion of fluid energy to electrical energy. More vibratory energy may be generated in the fluid than can be absorbed by the piezoelectric element. Consequently, such excess vibratory energy is wasted. In fact, at certain frequencies, such an impedance mismatch may cause dampening of the piezoelectric element vibrations with a resultant loss in output power.